Known types of communication systems include time division duplex (TDD) communication systems, such as radio TDD communication systems and optical TDD communication systems, wherein a bi-directional communication path is established between two sites, each of which has a transmitter and receiver, and wherein the two transmitters are synchronized so that only one is transmitting an information signal at any time. Such TDD systems may be a point to point system, wherein the system comprises only two sites, or a point to multi-point system, wherein the system comprises a "central" site which establishes a communication link to one of a plurality of other sites. An example of a point to multi-point TDD system is a wide area selective call radio system having a central site, wherein the central site has a fixed, high power transmitter and a fixed receiver, and has separate transmit and receive antennas located near each other. The point to multi-point TDD system of this example further has a plurality of transportable selective call radios, such as mobile radios or pagers, located within range of the central transmitter and receiver antennas. The selective call radios typically are transceivers (single unit transmitter/receivers), each of which has a single transmit/receive antenna, although some selective call radios have separate antennas for transmission and reception.
A high power transmit signal is used in wide area TDD systems in order to achieve long communication ranges. The high power transmit signal is typically used in the central site transmitter. A high power transmit signal can also be used in some types of radios, such as mobile radios, in wide area TDD systems. As described above, one of the two transmitters involved in the bi-directional TDD link is in a standby state while the other is on (transmitting). After a duplex time period, which is determined based on the type of information being communicated, the one transmitter starts transmitting and the other transmitter goes to the standby state. For example, a typical TDD digitized voice communication will have a duplex time period of approximately a millisecond, while a TDD data communication can have time periods much longer, such as 1875 milliseconds. When the power level transmitted in the on state is high, e.g., over 100 Watts, a problem arises in completely eliminating the radiation of power from the transmitter when the transmitter is in the standby state. Currently available, cost effective implementations of power control circuits in transmitters can achieve a 90 to 100 dB reduction of the power emitted from the on state to the standby state. Unfortunately, the residual power thus emitted by a high power transmitter in the standby state can still be sufficient to cause interference within a receiver between the residual signal received from the standby transmitter and an information signal being received from an on transmitter, even when the receiver is moderately far (e.g., a mile or two ) from the transmitter in the standby state.
Thus, what is needed is a means for reducing interference within a receiver between a residual signal from a first transmitter and an information signal from a second transmitter operating in a time division duplex communication system.